job(s) bless us! privileged operations on macos · anti-malware team, triage team iaronskaya....
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Job(s) Bless Us!Privileged Operations on macOS
@aronskaya 🇺🇦
WWCKyivmacOSChapterLead
SoftwareEngineer,Anti-malwareteam,Triageteam
iaronskaya
AgendaIntrotoprivilegedoperationsAPIonmacOS
FirstCleanMyMac'ssecurityissue,reportedby
CleanMyMacon
Comparisonofprivilegedoperationsimplementationonand
Summary&Takeaways
Intro to privileged operations API on macOS
Intr
o
High-level APIs
SMJobBless() AuthorizationExecuteWithPrivileges()
Intr
o
High-level APIs
SMJobBless() AuthorizationExecuteWithPrivileges()
Intr
o
High-level APIs
SMJobBless() AuthorizationExecuteWithPrivileges()
Intr
o
Intr
o
There is no ‘UnBless’ 😔
Signing requirements
👑🧢Client Privileged Helper
Client has requirements for Helper(s)
Helper has requirements for Client(s)
OS performs validation of the requirements ONLY on install & update of the Helper
NO validation is performed on establishing XPC connection
Intr
o
SMJobBless()1. Client has the Privileged Helper
executable in the bundle2. Signing requirements are met
• Both client and Helper are signed • Privileged Helper has a plist file for launchd
embedded into __TEXT section • Privileged Helper has Info.plist embedded • Client has signing requirements listed in its Info.plist
Intr
o
SMJobBless()3. Obtain Authorization object:
call AuthorizationCreate()
4. Call SMJobBless() with acquired Authorization object
Intr
o
SMJobBless()5. OS validates code signing requirements in client and helper’s
Info.plist and copies the executable from the bundle to /Library/PrivilegedHelperTools
Intr
o
SMJobBless()5. Client can establish XPC connection to the Privileged Helper
Intr
o
Apple’s Sample CodeIntr
o
Apple’s Sample CodeIntr
o
Apple’s Sample Code
🤔
Intr
o
Issue #1
🤨
Intr
o
First security issue,reported by
Talo
s
Zero-Day Reports• November 2018
Talo
s
Talo
s
Stumbled upon Talos’es Zero-Day reports
Contacted Talos for details, they answer the same day
We release a patched update
v. 4.2.0Talos reports insufficient fix
We release a patch v. 4.3.0
Tyler Bohan (Talos) delivers a talk
at OffenciveCon19
TimelineTalo
s
0 1
Tyler Bohan: ‘OSX XPC Revisited - 3rd Party Application Flaws’ at OffensiveCon19
https://www.youtube.com/watch?v=KPzhTqwf0bA
Talo
s
Tyler Bohan: ‘OSX XPC Revisited - 3rd Party Application Flaws’ at OffensiveCon19
https://www.youtube.com/watch?v=KPzhTqwf0bA
Talo
s
FixTalo
s
Fix #1Talo
s
on
h1
Timeline
March 2018
MacPaw launched a private h1 program for our other product Setapp
May 2019
CleanMyMac desktop client is added to the scope
h1
Client’s requirements
{
Bundle identifier Signing identity (team id)
{
h1
Client’s requirements
Privileged helper’s executable can be replaced with old version
{
Bundle identifier Signing identity (team id)
{
h1
What’s the fuss about old versions?h1
What’s the fuss about old versions?
El Capitan 10.11 Sierra 10.12 High Sierra 10.13 Mojave 10.14 Catalina 10.15
Hardened Runtime introduced in Mojave: • libraries signing validation == protect from dylib injection • remove get-task-allow from entitlements == protect from attaching with debugger
(and other things)
h1
Issue #2: stepsPreconditions: Privileged Helper is not authorized yet. A malicious executable is present on the user’s computer.
1. Download an app version, vulnerable to dylib injection
2. Replace the Privileged Helper executable in the installed app with the vulnerable one
3. User authorizes the Helper
4. Perform a dylib injection into the Helper—it is run as root!
h1
What about code signing?Replacing the Privileged Helper in the signed bundle doesn’t change anything, because
OS validates the signature only when app is quarantined
After the first launch no signature validation is performed on Mojave.
Time-to-time signature checks were announced in Catalina.
h1
Fix #2h1
{
Version check
Privileged Helper’s requirements
{
Signing identity (team id)
h1
old 🐞 client versions can connect
Privileged Helper’s requirements
{
Signing identity (team id)
h1
Issue #3: stepsPreconditions: Privileged Helper is authorized. A malicious executable is present on the user’s computer.
• Download an old app version, vulnerable to dylib injection
• Launch client executable with a dylib injection
• Call privileged helper’s methods from the injected code
• In our case it leads to LPE to root
h1
Issue #3: stepsPreconditions: Privileged Helper is authorized. A malicious executable is present on the user’s computer.
• Download an old app version, vulnerable to dylib injection
• Launch client executable with a dylib injection
• Call privileged helper’s methods from the injected code
• In our case it leads to LPE to root
Takeaway: Dylib injection does NOT break the code signature
h1
Fix #3h1
old 🐞 client versions can connect
other apps of the same vendor can connect
Privileged Helper’s requirements
{
Signing identity (team id)
h1
Fix #4h1
Privileged Helper’s codeh1
anyone can impersonate the client due to pid checks logic performed by OS
Privileged Helper’s codeh1
Issue #5: anyone can impersonate the client due to 🐞 racy 🐞 pid checks performed by OS
h1
h1
Fix #5h1
The APIs are private 😞h1
Privileged operations implementation on
and
Seta
pp
SMJobBless() AuthorizationExecuteWithPrivileges()
Application
API
# bugs reported* 5
Seta
pp
* as for March 2020
SMJobBless() AuthorizationExecuteWithPrivileges()
Application
API
# bugs reported* 5 0
* as for March 2020
Seta
pp
Summary & Takeaways
Summ
ary
Takeaways for developers1. Think about security in your project/company. A good start is creating a security@yourcompany.com email handle.
2. Have one source of truth for Client’s signing requirements and one for Privileged Helper’s, e.g. put them in Preprocessor Macros and use it in:
ℹ Info.plist file
👑 listener:shouldAcceptNewConnection:
3. In signing requirements check at least for:
🧑💻 signing identity
🆔 bundle identifier
#⃣ minimum version
4. In SecCodeCopyGuestWithAttributes use 🔖 audit token to obtain code reference for signature validation, not the pid
5. In order to be a good citizen remember to unregister the Privileged Helper via launchctl or SMJobRemove API, remove the executable from /Library/PrivilegedHelperTools and the auto generated .plist from /Library/LaunchDaemons
Summ
ary
Example set up requirementsfor Privileged Helper
1. Add User-Defined Build Settings:
CLIENT_REQUIREMENTS="@\"anchor trusted and certificate leaf [subject.CN] = \\\"$(CLIENT_SIGNING_IDENTITY)\\\" and info[CFBundleShortVersionString] >= \\\"$CLIENT_MIN_VERSION\\\" and identifier \\\"$CLIENT_IDENTIFIER\\\"\""
2. Use them to create a macro definition
Summ
ary
3. Use your Build Settings in Info.plist client requirements:
4. Use the Macro Definition from 2. in code to validate incoming connection:
Example set up requirementsfor Privileged HelperSu
mmar
y
📝 Summary/WishlistSumm
ary
📝 Summary/Wishlist1. We need the documentation
There is no easily available Apple’s documentation about securing XPC connection with Privileged Helpers
2. We need Code Samples
Apple’s code samples are not secure
3. Using pid to check the signature of a process is not secure. It should be clearly stated in docs
Checks by pid are racy by nature
4. Audit token should not be private
It is the most secure way, but it is not available to 3rd party developers
5. There should be some Uninstallation API
When the app is being removed, the Helpers are usually forgotten in /Library/PrivilegedHelperTools
Summ
ary
📖 Further Reading1. project-zero ‘Issue 1223: MacOS/iOS userspace entitlement checking is racy’ by
Ian Beer
2. OffensiveCon19 'OSX XPC Revisited - 3rd Party Application Flaws' by Tyler Bohan
3. Apple Developer Forums 'XPC restricted to processes with the same code signing?'
4. Objective Development ‘The Story Behind CVE-2019-13013’ by Christian from Little Snitch
5. ‘No Privileged Helper Tool Left Behind’ by Erik Berglund
Summ
ary
Call to Action 🧞If I could ask you to do 1 thing, let it be:
Summ
ary
Call to Action 🧞Summ
ary
reporting to Apple, that audit tokens should be made available for 3rd party developers:
If I could ask you to do 1 thing, let it be:
Thank you! 🤓
iaronskaya
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